Choosing not to archive redo logs significantly reduces your options for database
backups, as we??™ll discuss in Chapter 11, and is not advised by Oracle.
ARCHIVELOG
When Oracle rolls over to a new redo log, it archives the previous redo log. To
prevent gaps in the history, a given redo log cannot be reused until it is successfully
archived. The archived redo logs, plus the online redo logs, provide a
complete history of all changes made to the database. Together, they allow Oracle
to recover all committed transactions up to the exact time a failure occurred.
Operating in this mode enables tablespace and datafile backups.
The internal sequence numbers discussed earlier act as the guide for Oracle while it
is using redo logs and archived redo logs to restore a database.
ARCHIVELOG mode and automatic archiving
Starting with Oracle Database 10g, automatic archiving for an Oracle database is
enabled with the following SQL command:
ALTER DATABASE ARCHIVELOG
Deploying Physical Components | 47
If the database is in ARCHIVELOG mode, Oracle marks the redo logs for archiving
as it fills them.
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